Why Atelier Fabry-Pérot?

The heart of most lasers is the Fabry-Pérot etalon.

Portrait of Alfred Pérot

Alfred Pérot

Portrait of Charles Fabry

Charles Fabry

The etalon or Fabry-Pérot interferometer is named after its inventors, the 19th century French physicists Charles Fabry and Alfred Pérot. Fabry was also a co-discoverer of the ozone layer.

An interferometer is a measuring instrument that uses multiple reflections and constructive or destructive interference between the light waves to generate an interference pattern. Simple examples are found in most small iridescent rainbow patterns in nature - from oil films on water to the rainbows on freshly-sliced delicatessen meat.

Fabry and Pérot used their interferometer to filter the wavelength of light, allowing them to study its spectrum. Like Isaac Newton's prism, but far more precise.

Although it would be another 40 years before the 'optical maser', as it was first named, was considered, using their interferometer as part of the laser cavity was an obvious choice.

Licensed CC-by-sa by Stigmatella aurantiaca and Kevin j morse @Wikipedia

A Fabry–Pérot etalon. Light enters the etalon and undergoes multiple internal reflections.

Many people will have placed their head between two mirrors facing each other, and wondered if they can really see to infinity. If you do this, but instead of your head you use a box of something that amplifies light, then you have a laser. This arrangement with two facing mirrors is called an etalon.

Two graphs showing a comb filter function. Licensed CC-by-sa by DrBob@Wikipedia

The transmission of an etalon as a function of wavelength. A high-finesse etalon (red line) shows sharper peaks and lower transmission minima than a low-finesse etalon (blue).

Each reflection interferes with the others. Their sum effect depends on the relationship between the spacing between the mirrors and the wavelength of the light. In the laser resonator, it acts as a comb filter, allowing only a series of very narrow wavelengths to pass through. This is one of the factors giving lasers their single-colour monochromaticity. How effective the etalon is at selecting a narrow wavelength band is described as its 'finesse'.

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